The invention relates to a circuit arrangement for operating a discharge lamp, preferably a low pressure discharge lamp, with a high frequency inverter or frequency changer, in inductance connected series with the discharge lamp and a capacitance arranged parallel to the discharge lamp.
The operation of a discharge lamp includes all states of a discharge lamp from preheating to ignition and steady burning, i.e., combustion.
Low pressure discharge lamps, particularly fluorescent lamps, are used to a large extent for the generation of electric light. Compared to incandescent lamps they have a higher light intensity, a greater degree of efficiency and a longer service life.
Low pressure discharge lamps essentially comprise a discharge vessel, which in the case of fluorescent lamps is coated on the inside with a luminous substance, electrodes, a gas filling and a lamp socket with contact pins. With discharge lamps the light is generated by the process of a gas discharge in the discharge vessel.
Owing to its negative inner resistance, it is not possible to connect low pressure discharge lamps directly to a supply grid. Rather it is necessary to connect an auxiliary unit between the supply grid and low pressure discharge lamp so that this unit regulates the ignition and operation of the lamp.
There are various possibilities of creating auxiliary units which basically differ in the way in which the ignition of the lamp is carried out. Auxiliary units where the electrodes are preheated before the ignition of the gas are the most widespread.
In these types of auxiliary units the gas discharge is ignited by a voltage pulse. The conventional auxiliary units use a glow starter to produce voltage pulses. At the present time auxiliary units are preferably made with purely electronic components. This particularly applies to compact lamps wherein the auxiliary unit is integrated into the lamp socket within the smallest possible space. Compact lamps have small dimensions compared with the conventional tube-like fluorescent lamps.
Electronic auxiliary units are known. They basically comprise a low pass filter, a radio interference suppressor filter, a rectifier and a DC/AC or frequency converter. The DC/AC or frequency converter produces a high frequency alternating voltage of about 25 to 50 kHz which is applied to the electrodes of the discharge lamp. When a low pressure discharge lamp is operated with a high-frequency alternating voltage the lamps produce a higher light output than during low-frequency operation. Also the light produced with this type of operation is flicker-free.
German laid open application 3840845 A1 describes a circuit arrangement for operating a low pressure discharge lamp with an inductance connected in series with the discharge lamp and a capacitance arranged parallel to the discharge lamp. In this case, a two-pole is provided parallel to the inductance and is connected on one side to a switching point of the load circuit and on the other side via at least one diode to the positive pole and/or via a diode to the negative pole of a DC voltage source for supplying the circuit arrangement. The diodes cause a blockage of the preheating circuit after the ignition of the discharge lamp. With arc voltages of the discharge lamp of more than 70 V, the two-pole consists of a series circuit of a PTC thermistor and two Z-diodes polarized in the opposite flow-through direction. This serves to safeguard the blockage of the preheating circuit in case of arc voltages of more than 70 V.
From U.S. Pat. No. 4,647,820 a circuit arrangement is known for operating a discharge lamp which comprises a DC/AC converter for producing a high-frequency operating voltage for the discharge lamp, an inductance connected in series with the discharge lamp, a capacitance disposed parallel to the discharge lamp and a PTC thermistor disposed parallel to the capacitance. With this known circuit arrangement there is the disadvantage that even after ignition of the discharge lamp current flows through the PTC thermistor which accelerates aging of the PTC thermistor.
With the known circuit arrangement the starting characteristics of the discharge lamp are determined by the ambient temperature and the operating data of the PTC thermistor which change with age so that a constant ignition of a discharge lamp is not possible with the known circuit arrangements.